NATURE OF THE FACTORIAL TECHNIQUE 89

(iv) We can, if we think it helpful, take yet another step,
and suppose that both the test performances, and the forces
in terms of which they are to be described, are manifestations
of one and the same capacity for work, which can expend
itself in different directions. This single underlying
capacity it will be natural to call c energy.5 We can then
say that different tests, according to their differing difficulty,
will require different amounts of the same * mental energy j
irrespective of their specific quality or kind ; and we can go
on to declare that different individuals must possess a
different amount of c mental power,' defining power in the
usual way as the rate at which energy is expended, and
measuring it in terms of the amount of work accomplished
per unit of time.

But it now becomes evident, I think, that the energy
which we thus postulate is a purely logical construction,
and has no more than a conventional significance : we can
hardly treat it as " in the last resort presumably identifiable
with the neural, neuro-muscular, or neuro-endocrino-
muscular energy of the organism," or with the " energy
that serves the whole cortex or possibly the whole nervous
system " (cf, Spearman [56], pp. 121 f.). Certainly, its amount
cannot be identified with, or even directly related to, the
amount of physico-chemical energy required for the propa-
gation of nerve-currents. No doubt, during mental work
the heat production of the central nervous system (exceed-
ingly minute in any case) is somewhat increased; but, so
far as we can discover, the increase is at least as great during
unintelligent physical activity, and probably far greater
during emotional excitement. If we include other sources
of physiological energy—the muscles and the endocrine
glands—the lack of correspondence between physical work
and mental work is still more flagrant.1 All these physi-
ological forms of energy have their mechanical equivalent:

1 In such forms of work as sawing wood, the extra calories per hour may
rise to 200 or 300. But with " strong mental effort expended in solving
mathematical problems " Benedict found an increase of only 3 or 4 per cent.
He adds: " the cloistered scholar at his books may be surprised to learn that
the extra calories needed for one hour of intense intellectual effort would be
completely met by eating one half of awaited pea-nut,"